One process for synthesizing perfectly spherical polyamide-12 powders consists in dissolving lauryllactam at 140° C. in a liquid paraffin comprising potassium stearate and in then initiating the polymerization by adding potassium lactamate and phosphorus trichloride (J61-233.019 and J72-024.960). This method is adapted to the synthesis of perfectly spherical powders formed of copolyamides obtained from lauryllactam and from one or more other lactams, such as caprolactam (J72-025.157). The powders obtained by this process have virtually no porosities.
The industrial preparation of porous polyamide particles, in particular spheroidal porous polyamide particles, with a narrow particle size distribution by anionic polymerization of lactam(s) in suspension (FR 1 213 993, FR 1 602 751) or in solution (DE 1 183 680) in an organic liquid is known. The processes described in these patents make it possible to directly obtain polyamide particles, which separate by themselves from the liquid medium as they are formed. The organic solvents employed for the anionic polymerization are generally chosen from aliphatic and/or aromatic hydrocarbons, alone or as a mixture (for example, hydrocarbon fractions), their boiling range generally lying between 140 and 170° C. The lactam or the mixture of lactams is initially completely dissolved in the solvent or the mixture of solvents in the presence of other ingredients, such as, for example, an inorganic or organic filler. As is taught in patents FR 1 213 993, FR 1 521 130 and EP 192 515, the mean diameter of the particles is regulated using processing parameters which are the stirring speed, the measured addition in successive portions of the reactants or the weight of filler introduced. As taught in patents EP 303 530 and EP 192 515, the filler which is added acts as crystallization seeds.
For some applications, such as, for example, for coil coating, it is necessary to obtain polyamide or copolyesteramide particles having a mean diameter of greater than 40 μm, indeed even of greater than 60 μm. We have tested finely divided fillers which generate crystalline seeds (example: pyrogenic silica) under the conditions of EP 192 515 while reducing the stirring speed and/or while reducing the amount of seeds introduced. However, these actions do not make it possible to reproducibly obtain particles with a diameter of greater than 40 μm and it appears to be virtually impossible to obtain particles with a diameter of greater than 60 μm. In this case, the stirring speed cannot be reduced excessively severely as it is necessary to ensure, in spite of everything, that the reaction medium is homogeneous. The reduction in the amount of filler does not make it possible either to increase the diameter as impurities, which certainly act as disrupting seeds, may be present in an excessively large amount in the reaction medium.
The applicant company has found that, in order to solve this technical problem and to obtain polyamide or copolyesteramide particles with a narrow particle size distribution and with a mean diameter of between 40 and 150 μm, preferably between 60 and 100 μm, it is necessary to introduce, into the polymerization medium, an inorganic filler with a mean diameter of between 1 and 30 μm, preferably between 2 and 20 μm, advantageously between 3 and 11 μm and more advantageously still between 4 and 8 μm.
Thus, the introduction of said filler makes it possible to reduce the effect of the impurities which would be present in the reaction medium and makes it possible to avoid an excessively severe reduction in the stirring speed. This type of inorganic filler thus ensures the production of polyamide or copolyesteramide particles with a mean diameter of greater than 40 μm, preferably of greater than 60 μm.
Patent EP 192 515 describes a process for the production of polyamide particles by an anionic polymerization of lactam(s) in solution in the presence of a finely divided organic or inorganic filler. The filler can be finely divided and dehydrated silica.
Patent EP 303 530 describes a process for the production of polyamide particles with a mean diameter of between 1 and 20 μm by an anionic polymerization of lactam(s) in a solvent in which the lactam(s) is in a state of supersaturation. It is possible to introduce, into the reaction medium, crystallization seeds which are provided in the form of a finely divided filler which can be silica or talc.
In patent FR 1 601 195, the polymerization of the lactam in solution is carried out in the presence of a preformed polyamide powder.
In patent FR 1 213 993, the polymerization of the lactam in solution is carried out without agglomeration of the polyamide on the walls of the reactor in the presence of a preformed polyamide powder. Pigments, such as carbon black or titanium dioxide, can also be added initially.
In all these patents, the filler which is introduced into the polymerization medium is composed of finely divided inorganic particles or of polyamide powder.
Patent EP 1 172 396 describes a process for the production of copolyesteramide particles by copolymerization of two lactams and of a lactone in the presence of a finely divided inorganic or organic filler, the mean diameter of which is between 0.01 and 10 μm.
None of these documents describes that the use of an inorganic filler with a mean diameter of between 1 and 30 μm makes it possible to obtain polyamide or copolyesteramide particles with a mean diameter of between 40 and 150 μm.